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Venus

Cloud-covered Venus is the second planet from the Sun, and passes closer to Earth than any other planet. Even so, Venus remained shrouded in mystery until the Space Age because the clouds hide its surface from view. Early planetary scientists envisioned a lush world of oceans, constant rains, and abundant life. Yet the reality is far different: Venus is hot, dry, and totally unsuitable for life.

The Venusian clouds kept astronomers from seeing the planet's surface. Without visible landmarks, they could not measure how fast Venus turns on its axis. It took cloud-penetrating radar, first aimed at Venus in the early 1960s, for scientists to discover that Venus rotates backward as compared to the other planets in the solar system. This means that the Sun rises in the west and sets in the east. And Venus' rotation is extremely slow, so a "day" on Venus is longer than a year.

The planet's "retrograde" rotation could be the result of a collision between Venus and a large proto-planet billions of years ago, when the solar system was forming.

Early spacecraft discovered that Venus' atmosphere is thick and toxic. It is made primarily of carbon dioxide, with a surface pressure 90 times greater than on Earth -- equivalent to the pressure at a depth of more than a half-mile (900 meters) below the surface of the ocean. Clouds of sulfuric acid, blown by high-speed winds, top the atmosphere.

This dense atmosphere makes Venus the hottest planet in the solar system. The atmosphere absorbs the Sun's heat but doesn't allow it to escape. This "greenhouse effect" has heated Venus' surface to about 860 degrees Fahrenheit (450°C) — hot enough to melt lead.

The first views of the surface came from four Soviet landers, each of which survived for only a few minutes before succumbing to the heat and pressure. The images showed flat, angular rocks around the landers, but not much else.

The best view of Venus came from the Magellan spacecraft, which entered orbit around Venus in 1990. Its cloud-penetrating radar mapped 98 percent of the planet's surface. Magellan discovered mountains on Venus that are taller than any on Earth, as well as a valley that is the longest and deepest in the solar system. Thousands of volcanoes dot the Venusian surface. Magellan provided hints that some of them may still be active, and later orbiters discovered evidence of active eruptions. Magellan's observations suggest that the planet's entire surface was repaved by global volcanic eruptions about 500 million years ago.

The only impact craters detected on Venus are large. The surface lacks smaller craters because small meteoroids burn up in the planet's thick atmosphere before they hit the ground.

Although Venus is completely dry today, a spacecraft in the 1980s found evidence that the planet could have been much wetter in the distant past. In fact, it might have had enough water to cover the entire planet to a depth of 50 feet or more.

Scientists think Venus' interior is a lot like Earth's, with an iron-nickel core surrounded by a rocky mantle. But unlike Earth, Venus does not have tectonic plates or a magnetic field.

Viewing Guide

♀The brilliant planet starts 2019 as the Morning Star, shining before and during dawn twilight until July. It then disappears from view as it passes behind the Sun. It returns to view as the Evening Star in late September, and remains in the morning sky through the end of the year.

Exploration

The Soviet Venera 9 became the first spacecraft to transmit pictures from the surface of another planet when it touched down on Venus in 1975. And in a mission that began 15 years later, the American Magellan orbiter used cloud-penetrating radar to map more than 98 percent of Venus' surface. Its images revealed hundreds of volcanoes, giant lava flows, cracked domes of volcanic rock, and long canyons. Europe's Venus Express made extensive measurements of the planet's atmosphere, discovering that lightning crackles in some of the clouds and finding evidence of active volcanic eruptions.

0.91 that of Earth (If you weigh 100 pounds, you would weigh about 91 pounds on Venus.)

Known Moons

None

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The Astro Guides for the Solar System and Beyond the Solar System are supported by the National Aeronautics and Space Administration under Grant Nos. NNG04G131G and NAG5-13147, respectively.